fix(ngModel): deregister from the form on scope not DOM destruction

Due to animations, DOM might get destroyed much later than scope and so the element $destroy event
might get fired outside of $digest, which causes changes to the validation model go unobserved
until the next digest. By deregistering on scope  event, the deregistration always happens
in $digest and the form validation model changes will be observed.

Closes #4226
Closes #4779